1. Field of the Invention
This invention relates to a process for producing a magnet of strontium ferrite having high performance from iron oxides obtained from iron are of magnetite or mill scale.
2. Description of the Prior Art
A magnet of strontium ferrite (SrO.nFe.sub.2 O.sub.3, where n is from 5.0 to 6.2) is produced by a process including the steps of mixing iron oxide obtained from iron ores or mill scale with strontium oxide or carbonate, calcining their mixture, pulverizing the calcined material, molding the pulverized material into a particular shape, and sintering the molded material. When the mixture is calcined, the iron oxide and the strontium oxide or carbonate react with each other to form a ferrite.
The iron ores and mill scale, however, contain a large amount of FeO or Fe.sub.3 O.sub.4 in addition to Fe.sub.2 O.sub.3, as is obvious from TABLE 1 showing the composition of each of magnetite and hematite, which are used as iron ores, and mill scale. This necessitates the oxidation of iron ore of magnetite or mill scale in an atmosphere containing oxygen, or in the air to convert it to a source of exclusive supply of Fe.sub.2 O.sub.3 before mixing it with the strontium oxide or carbonate. The oxidized magnetite or mill scale has, however, so large an average particle diameter that its pulverizing requires a great deal of time, resulting undesirably in a high cost of production and a low level of productivity.
TABLE 1 ______________________________________ (wt. %) Iron ores T.Fe FeO Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 SiO.sub.2 Al.sub.2 O.sub.3 MgO CaO ______________________________________ magnetite 69.1 -- 1.5 93.7 1.5 0.30 0.60 0.61 Hematite 68.08 0.06 97.26 -- 0.41 0.07 0.06 0.03 Mill 74.88 67.5 32.6 -- 0.09 0.06 0.003 0.10 scale ______________________________________
I, the inventor of this invention, have, therefore, examined the cross-sectional structure of oxidized magnetite and mill scale, and found that it consists of a surface layer of Fe.sub.2 O.sub.3 having a depth of six microns or below, and an inner layer composed of a mixture of FeO and Fe.sub.3 O.sub.4. I have concluded that it is due to this structure that the oxidized material requires a great deal of crushing time, and that when it is calcined, the iron oxide and the strontium oxide or carbonate fail to undergo a complete ferritizing reaction.
There is a great demand for a magnet of ferrite having a high level of performance which can be used in an electric motor for an automobile. There is, however, not known any magnet of this type having satisfactory properties. The known magnets have only a value of Br ranging from 4.0 to 4.2 kG, a value of Hc ranging from 3.3 to 3.7 kOe, a value of (BH).sub.max ranging from 3.8 to 3.9 MGOe, and a value of iHc ranging from 3.6 to 3.9, at best.